U.S. patent application number 17/690528 was filed with the patent office on 2022-06-23 for sintered body, method for producing same, and dielectric composition.
The applicant listed for this patent is Murata Manufacturing Co., Ltd., National University Corporation Hokkaido University. Invention is credited to Akira HOSONO, Masashi INOGUCHI, Shinichi KIKKAWA, Yuji MASUBUCHI.
Application Number | 20220194861 17/690528 |
Document ID | / |
Family ID | |
Filed Date | 2022-06-23 |
United States Patent
Application |
20220194861 |
Kind Code |
A1 |
MASUBUCHI; Yuji ; et
al. |
June 23, 2022 |
SINTERED BODY, METHOD FOR PRODUCING SAME, AND DIELECTRIC
COMPOSITION
Abstract
A sintered body containing polycrystalline grains of a metal
oxynitride containing at least two metal elements, wherein Ba and
at least one metal element of a crystal phase of the sintered body
are contained in a triple point that is not a void between the
polycrystalline grains. A method for producing the sintered body
includes sintering a mixture of at least a metal oxynitride as a
main component and a sintering aid containing cyanamide in an
atmosphere containing nitrogen or a rare gas or in a
reduced-pressure atmosphere of 10 Pa or less while applying a
mechanical pressure with a retention time at a maximum heating
temperature during the sintering set to 1 minute to 10 minutes.
Inventors: |
MASUBUCHI; Yuji;
(Sapporo-shi, JP) ; HOSONO; Akira; (Sapporo-shi,
JP) ; KIKKAWA; Shinichi; (Osaka, JP) ;
INOGUCHI; Masashi; (Nagaokakyo-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
National University Corporation Hokkaido University
Murata Manufacturing Co., Ltd. |
Sapporo-shi
Nagaokakyo-shi |
|
JP
JP |
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|
Appl. No.: |
17/690528 |
Filed: |
March 9, 2022 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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PCT/JP2020/034471 |
Sep 11, 2020 |
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17690528 |
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International
Class: |
C04B 35/58 20060101
C04B035/58; C04B 35/645 20060101 C04B035/645; C04B 35/63 20060101
C04B035/63; B01J 27/24 20060101 B01J027/24; B01J 35/00 20060101
B01J035/00; B01J 37/08 20060101 B01J037/08; G01N 33/00 20060101
G01N033/00; H01G 4/12 20060101 H01G004/12 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 12, 2019 |
JP |
2019-166041 |
Claims
1. A sintered body comprising: polycrystalline grains of a metal
oxynitride containing at least two metal elements, wherein Ba and
at least one metal element of a crystal phase of the sintered body
are contained in a triple point that is not a void between the
polycrystalline grains.
2. The sintered body according to claim 1, wherein a concentration
ratio between the Ba and the metal element in the triple point is
higher than a concentration ratio between Ba and the metal element
in the polycrystalline grains.
3. The sintered body according to claim 1, wherein the sintered
body contains a portion where the polycrystalline grains are in
surface contact with each other.
4. The sintered body according to claim 3, wherein a grain boundary
portion where the polycrystalline grains are in surface contact
with each other does not have an amorphous phase.
5. The sintered body according to claim 1, wherein an equivalent
circle average size of voids in the sintered body is 1.0 .mu.m or
less.
6. The sintered body according to claim 1, wherein an equivalent
circle diameter of a region excluding voids in a portion of the
sintered body other than the polycrystalline grains is 1.0 .mu.m or
less.
7. The sintered body according to claim 1, wherein a degree of
densification of the oxynitride grains in the sintered body is 55%
or more.
8. The sintered body according to claim 1, wherein 90% or more of a
crystal phase of the oxynitride, estimated from a diffraction peak
intensity ratio of an X-ray diffraction pattern, is a perovskite
structure represented by AB(O,N).sub.3.
9. The sintered body according to claim 1, the sintered body
comprises 0 wt % to 10 wt % of a composite metal oxynitride phase
represented by A.sub.2BO.sub.4-xNx, where x is greater than 0.
10. The sintered body according to claim 1, wherein a crystalline
metal oxide phase, a metal carbide phase, and a metal oxynitride
phase are not contained in the sintered body.
11. The sintered body according to claim 10, wherein the composite
metal oxynitride phase contains at least one of an alkaline earth
metal or La as a constituent element thereof.
12. The sintered body according to claim 11, wherein the alkaline
earth metal is at least one of Ba, Sr, or Ca, and the composite
metal oxynitride phase contains at least one of Ba, Sr, Ca, or
La.
13. The sintered body according to claim 10, wherein the composite
metal oxynitride contains Ta as a constituent element thereof.
14. The sintered body according to claim 1, wherein, in a diffuse
reflection spectrum of a powder obtained by pulverizing the
sintered body, a difference between a maximum value and a minimum
value in a wavelength range of 400 nm to 800 nm is 10% to 50%.
15. The sintered body according to claim 1, wherein a
direct-current volume resistance in the sintered body is 10.sup.6
.OMEGA.cm or more.
16. A dielectric composition comprising the sintered body according
to claim 1, wherein the sintered body has a relative permittivity
of 200 or more when an electric field of 100 Hz to 1 MHz is applied
in a temperature range of -50.degree. C. to 150.degree. C.
17. A dielectric composition comprising the sintered body according
to claim 1, wherein the sintered body has a rate of change in
relative permittivity within .+-.10% when an electric field of 100
kHz is applied due to a temperature change in a temperature range
of 30.degree. C. to 150.degree. C.
18. A capacitor comprising: the dielectric composition according to
claim 16; and at least a pair of electrodes facing each other with
the dielectric composition interposed between the pair of
electrodes.
19. A photocatalytic composition comprising the sintered body
according to claim 1.
20. A photoelectric conversion element comprising the sintered body
according to claim 1.
21. A gas sensor comprising the sintered body according to claim
1.
22. A method for producing a sintered body, the method comprising:
sintering a mixture of at least a metal oxynitride as a main
component and a sintering aid containing cyanamide in an atmosphere
containing nitrogen or a rare gas or in a reduced-pressure
atmosphere of 10 Pa or less while applying a mechanical pressure
with a retention time at a maximum heating temperature during the
sintering set to 1 minute to 10 minutes.
23. The method for producing a sintered body according to claim 22,
wherein the cyanamide is BaCN.sub.2.
24. The method for producing a sintered body according to claim 22,
wherein the sintering is performed at a temperature of 880.degree.
C. to 950.degree. C.
25. The method for producing a sintered body according to claim 22,
wherein the sintering aid is mixed at a proportion of 3 wt % to 10
wt % with respect to 100 wt % of the metal oxynitride.
26. The method for producing a sintered body according to claim 22,
wherein the sintering aid is a powder or a particle.
27. The method for producing a sintered body according to claim 22,
wherein a temperature raising rate at the time of the sintering is
50.degree. C./min to 100.degree. C./min.
28. The method for producing a sintered body according to claim 22,
wherein the mixture is a powder in which the metal oxynitride and
the sintering aid containing cyanamide are mixed, and the sintering
is conducted in a state where the powder is in contact with a
composite metal oxynitride or boron nitride having a different
cyanamide content.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a continuation of International
application No. PCT/JP2020/034471, filed Sep. 11, 2020, which
claims priority to Japanese Patent Application No. 2019-166041,
filed Sep. 12, 2019, the entire contents of each of which are
incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The present invention relates to a sintered body composed of
polycrystalline grains of a metal oxynitride, a dielectric
composition containing the sintered body, and a method for
producing the sintered body.
BACKGROUND OF THE INVENTION
[0003] In recent years, sintered bodies of a metal oxynitride
having a perovskite structure have been known as dielectric
materials. For example, Patent Document 1 below discloses a
sintered body containing a complex of a plurality of crystal grains
containing a metal oxynitride and an amorphous substance. This
amorphous substance is present at an interface between crystal
grains. The amorphous substance contains carbon and nitrogen. In
the method for producing a sintered body described in Patent
Document 1, a metal oxynitride and a sintering aid containing
cyanamide are sintered in an atmosphere containing nitrogen in a
state of being in contact with each other. As the cyanamide, barium
cyanamide (BaCN.sub.2) is preferably used.
[0004] Patent Document 1: WO 2018/173491
SUMMARY OF THE INVENTION
[0005] In the sintered body containing a metal oxynitride and the
method for producing the sintered body described in Patent Document
1, the nitrogen content is sufficiently large, and therefore a
sintered body having excellent dielectric properties and the like
can be obtained.
[0006] However, the proportion of the oxynitride in the obtained
sintered body was not sufficiently increased, and thus physical
properties such as dielectric properties cannot be sufficiently
utilized. Moreover, relatively large voids are generated in the
sintered body, and a dense sintered body cannot be obtained.
[0007] An object of the present invention is to provide a sintered
body containing a denser metal oxynitride and a method for
producing the sintered body.
[0008] The sintered body according to the present invention
contains polycrystalline grains of a metal oxynitride containing at
least two metal elements, wherein barium (Ba) and at least one
metal element of a crystal phase of the sintered body are contained
in a triple point that is not a void between the polycrystalline
grains.
[0009] In addition, a method for producing a sintered body
according to the present invention includes sintering a mixture of
at least a metal oxynitride as a main component and a sintering aid
containing cyanamide in an atmosphere containing nitrogen (N.sub.2)
or a rare gas or in a reduced-pressure atmosphere of 10 Pa or less
while applying a mechanical pressure with a retention time at a
maximum heating temperature during the sintering set to 1 minute to
10 minutes.
[0010] According to the sintered body and the method for producing
the sintered body according to the present invention, it is
possible to provide a sintered body containing a more dense metal
oxynitride.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 is a graph showing an X-ray diffraction (XRD) pattern
at each depth position from a surface to a bottom surface of a
sintered body obtained in Example 1.
[0012] FIG. 2 is a scanning electron microscope (SEM) photograph
(magnification: 5,000 times) of an outer peripheral side portion of
the sintered body obtained in Example 1.
[0013] FIG. 3 is an SEM photograph (magnification: 5,000 times) of
the inside of the sintered body obtained in Example 1.
[0014] FIG. 4 is a photograph showing an image of the sintered body
obtained in Example 1, observed with a high-angle annular
dark-field scanning transmission electron microscope (HAADF-STEM)
(magnification: 160,000 times).
[0015] FIG. 5 is a photograph showing a bright field image of the
sintered body obtained in Example 1, observed a with transmission
electron microscope (TEM).
[0016] FIG. 6 is an image of the sintered body obtained in Example
1 observed with a HAADF-STEM, and is a photograph showing positions
where the concentration ratio between Ba and tantalum (Ta) is
measured.
[0017] FIG. 7 is a photograph showing a TEM bright field image of
the sintered body obtained in Example 1.
[0018] FIG. 8 is a photograph showing a TEM bright field image of
the sintered body obtained in Example 1.
[0019] FIG. 9 is a photograph showing a TEM bright field image of
the sintered body obtained in Example 1.
[0020] FIG. 10 is a photograph showing a TEM bright field image of
the sintered body obtained in Example 1.
[0021] FIG. 11 is a photograph showing a TEM bright field image of
the sintered body obtained in Example 1.
[0022] FIG. 12 is a graph showing the distribution of equivalent
circle diameter of crystal grains of the sintered body obtained in
Example 1, measured from FIG. 7 to FIG. 11.
[0023] FIG. 13 is a graph showing the distribution of equivalent
circle diameter of voids of the sintered body obtained in Example
1, measured from FIG. 7 to FIG. 11.
[0024] FIG. 14 is a graph showing the distribution of equivalent
circle diameter of a portion considered to be an amorphous phase of
the sintered body obtained in Example 1, measured from FIG. 7 to
FIG. 11.
[0025] FIG. 15 is an SEM photograph of the sintered body obtained
in Example 1 observed at a magnification of 300 times.
[0026] FIG. 16 is an SEM photograph of the sintered body obtained
in Example 1 observed at a magnification of 5,000 times.
[0027] FIG. 17 is a graph showing the diffuse reflection spectrum
of a powder obtained by pulverizing the sintered body obtained in
Example 1, measured in a wavelength range of 400 nm to 800 nm.
[0028] FIG. 18 is a graph showing the relationship between the
temperature and the volume resistance of the sintered body obtained
in Example 1, measured at a plurality of frequencies in a range of
100 Hz to 1 MHz and a temperature in a range of -50.degree. C. to
150.degree. C.
[0029] FIG. 19 is a graph showing the relationship between the
temperature and the relative permittivity (.epsilon.r) of the
sintered body obtained in Example 1, measured at a frequency of 100
kHz and a temperature in a range of 30.degree. C. to 160.degree.
C.
[0030] FIG. 20 is a graph showing the relationship between the
temperature and the dielectric loss (tan .delta.) of the sintered
body obtained in Example 1, measured at a frequency of 100 kHz and
a temperature in range of 30.degree. C. to 160.degree. C.
[0031] FIG. 21 is a graph showing the relationship between the
temperature and the relative permittivity (.epsilon.r) of the
sintered body obtained in Example 1, measured at a frequency of 100
Hz to 1 MHz and temperatures of -50.degree. C., 50.degree. C., and
150.degree. C.
[0032] FIG. 22 is a graph showing the relationship between the
temperature and the dielectric loss (tan .delta.) of the sintered
body obtained in Example 1, measured at a frequency of 100 Hz to 1
MHz and temperatures of -50.degree. C., 50.degree. C., and
150.degree. C.
[0033] FIG. 23 is a graph showing the diffuse reflection spectra of
a sintered body obtained in Example 2 and a powder obtained by
pulverizing the sintered body, measured in a wavelength range of
400 nm to 800 nm.
DETAILED DESCRIPTION OF THE INVENTION
[0034] Hereinafter, the present invention will be clarified by
describing specific embodiments of the present invention with
reference to the drawings.
[0035] As a result of intensive studies on the above-mentioned
problems, the inventors of the present application have found that
the sintered body according to the present invention having higher
denseness can be obtained by performing sintering in an atmosphere
containing N.sub.2 or a rare gas or in a reduced-pressure
atmosphere of 10 Pa or less while further applying a mechanical
pressure, in a state where at least a metal oxynitride as a main
component and a sintering aid containing cyanamide are mixed, with
the retention time at the maximum heating temperature during
sintering set to a short time of 1 minute to 10 minutes. The
sintered body according to the present invention contains
polycrystalline grains of a metal oxynitride containing at least
two metal elements, wherein Ba and at least one metal element of a
main crystal phase of the sintered body are contained in a triple
point that is not a void between the polycrystalline grains. Here,
the triple point is a grain boundary located at a portion where
three crystal grains of the polycrystalline grains are in contact
with each another. In the sintered body according to the present
invention, the triple point contains Ba and at least one metal
element constituting a main crystal phase of the sintered body. As
a result, the number of voids in the grain boundary portion is
small, and the denseness is enhanced. As described above, such a
structure is realized by performing sintering in a short time while
applying a mechanical pressure.
[0036] Hereinafter, details of the present invention will be
described.
[0037] In the method for producing a sintered body described in
Patent Document 1, the denseness of the sintered body was not
sufficient as described above. This is considered to be due to the
following reason. In the production method described in Patent
Document 1, a heat treatment is performed after a cyanamide-based
sintering aid is mixed with an oxynitride. Therefore, it has been
considered that thermal decomposition of the oxynitride phase and
occurrence of nitrogen deficiency can be suppressed. This is
because BaCN.sub.2 melts at a temperature around 900.degree. C.,
which is lower than a temperature at which nitrogen (N) contained
in the oxynitride is partially released from the oxynitride, and
the resulting liquid phase can dissolve the oxynitride phase. The
dissolved oxynitride phase is reprecipitated to cause grain
growth.
[0038] However, in order to cause dissolution and reprecipitation
of the oxynitride phase, it is required to entirely cover the
surface of the oxynitride grains with the melt of BaCN.sub.2. In
order to realize such a state, it is necessary to mix a large
amount of BaCN.sub.2.
[0039] When BaCN.sub.2 is melted by heating, BaCN.sub.2 as a liquid
phase penetrates into gaps in the molded body due to a capillary
phenomenon. Conversely, the portions where the grains of BaCN.sub.2
were originally present become voids. As a result, as shown in FIG.
3 of Patent Document 1, huge voids having a diameter of several
tens of .mu.m to several hundred .mu.m may be generated in the
resulting sintered body.
[0040] In addition, Patent Document 1 describes that the degree of
densification of the sintered body obtained by the production
method described in Patent Document 1 is about 80%, but the dense
portion is in a state where oxynitride grains and a large amount of
BaCN.sub.2 solidified phase are mixed. The net degree of
densification of the oxynitride grains is several tens of percent
lower than 80% accordingly. Therefore, physical properties such as
electrical characteristics derived from the oxynitride cannot be
sufficiently utilized.
[0041] On the other hand, in the sintered body according to the
present invention, since the sintered body is sintered in a state
where a mechanical pressure is applied as described above, the
denseness of the oxynitride is dramatically improved.
[0042] The sintered body according to the present invention
contains polycrystalline grains of a metal oxynitride having at
least two metal elements, wherein Ba and at least one metal element
of a crystal phase of the sintered body are contained in a triple
point that is not a void between three crystal grains of the
polycrystalline grains.
[0043] As the at least two metal elements contained in the metal
oxynitride, an alkaline earth metal or La is suitably used. By
using at least one of an alkaline earth metal or lanthanum (La), a
sintered body having a high nitrogen content can be easily
obtained. As the alkaline earth metal, Ba, strontium (Sr), and
calcium (Ca) are preferable, and it is preferable to use at least
one of four types of metal elements including La. That is, it is
preferable to use at least one of Ba, Sr, Ca, or La. More
preferably, at least one of Ba or Sr is suitably used. When Ba or
Sr is used, a sintered body having a higher nitrogen content can be
reliably obtained. A plurality of crystal grains containing a metal
oxynitride are crystalline. The sintered body according to the
present invention contains a plurality of the crystalline
grains.
[0044] In the sintered body, the average value of the equivalent
circle diameters of the crystal grains is preferably 0.10 .mu.m or
more. In that case, it can be considered that the crystal grains
are formed by bonding and growing oxynitride grains as a raw
material of the sintered body. When very fine crystal grains of
0.10 .mu.m or less are used, the surface area of crystal grains to
be covered with molten BaCN.sub.2 at the time of sintering becomes
too large, and thus it is necessary to mix a large amount of
BaCN.sub.2 as in Patent Document 1. In addition, the average value
of the equivalent circle diameters of the crystal grains is
desirably 1.0 .mu.m or less. When large crystal grains of 1.0 .mu.m
or more are used, relatively coarse voids of 1.0 .mu.m or more are
likely to remain even when sintering is performed while applying a
mechanical pressure. This causes disadvantages that electrical
characteristics such as dielectric properties of an oxynitride
cannot be utilized, and mechanical strength is likely to decrease.
The average value of the equivalent circle diameters can be
obtained by using image analysis software "A-Zou Kun" (manufactured
by Asahi Kasei Engineering Corporation) on an image acquired at a
magnification at which the shape and size of the grain can be
determined (for example, an image observed with a transmission
electron microscope at a magnification of 100,000 to 200,000
times).
[0045] A method of analyzing the average value of equivalent circle
diameters using the above "A-Zou Kun" will be described.
[0046] First, an image at a magnification at which the shape of the
grains and voids in the sintered body can be distinguished was
acquired with a microscope. In the present invention, an image at a
magnification of 100,000 to 160,000 times was acquired with a
transmission electron microscope. Subsequently, the brightness and
contrast were adjusted so that the shape of the grains, the
boundaries between the grains, and the boundaries between the
grains and the void portion were conspicuous. A binarization
treatment was performed, and only the grain portion was
extracted.
[0047] Note that if the "color extraction" of "A-Zou Kun" described
above was not complete, the "color extraction" was manually
supplemented.
[0048] When a portion other than the grains, that is, an amorphous
portion or a void portion in the sintered body was extracted, this
was deleted.
[0049] The number, area, and equivalent circle diameter of the
grains were measured by "Particle Analysis" of image processing
software.
[0050] Note that the number, area, and equivalent circle diameter
of the void portion regarded as a grain can be measured by
extracting only the void portion by the binarization treatment. In
addition, a portion having an intermediate color tone in the image,
which is neither a grain portion having a dark color tone nor a
void portion having a bright color tone, can be extracted by
manually compensating the contour thereof, and similarly, the
number, area, and equivalent circle diameter thereof can be
measured.
[0051] The crystal grain includes a perovskite structure.
[0052] The sintered body according to the present invention has a
high relative permittivity. Therefore, the sintered body of the
present invention is suitably used for a dielectric
composition.
[0053] As described above, the reason why the sintered body
according to the present invention is excellent in characteristics
as a dielectric is the following reasons. Oxygen (O) and N are
sufficiently contained in the sintered body, and lattice defects of
O or N are adequately small. As a result, the insulating properties
are improved, and the degree of densification of the sintered body
is high, and the proportion of the oxynitride crystal grains in the
volume of the sintered body is high. As described in Examples
described later, the sintered body obtained in the present
invention exhibits orange to red almost similarly to general
oxynitride powders. Therefore, it can be seen that desorption of N
hardly occurs, and a sintered body having a large N content is
obtained.
[0054] The color tone of orange to red indicates that the band gap
is in the visible light region. Therefore, the sintered body
according to the present invention can be suitably used for a
photocatalytic composition, which responds to visible light, a
photoelectric conversion element, and the like. In addition, the
element exposed on the outermost surface of the sintered body may
contain N unlike the conventional oxide sintered body. This
provides a possibility that the reaction to the contacted gas is
different from that of the oxide. Therefore, a gas sensor capable
of measuring a gas that cannot be measured by a conventional oxide
sensor can also be provided.
[0055] In the polycrystalline grains of the metal oxynitride having
at least two types of metal elements, a portion where three crystal
grains are in contact with each another is a triple point. An
example of the triple point will be described with reference to
photographs of FIG. 4 and FIG. 6 showing the results of Examples
described later. FIG. 4 is a HAADF-STEM photograph (magnification:
160,000 times) of the sintered body obtained in Example 1. FIG. 6
is a photograph showing the main part in an enlarged manner. As
shown in FIG. 4, a large number of crystal grains are in contact
with each other. As shown in an enlarged view in FIG. 6, a triple
point exists at an interface between the three crystal grains. In
FIG. 6, reference numerals 1 to 6 denote a measurement position
described later. A portion indicated by a measurement position 4 in
FIG. 6 is a triple point. In the sintered body of the present
invention, the triple point contains Ba and at least one metal
element constituting the crystal phase. Preferably, the
concentration ratio between Ba and the metal element in the triple
point is higher than the concentration ratio between Ba and the
metal element in the crystal grain.
[0056] That is, the measurement positions 1, 2, 3, 5, and 6 shown
in FIG. 6 are measurement positions in the crystal grain, whereas
the measurement position 4 indicated by the numeral 4 is a
measurement position in the triple point. The Ba concentration, the
Ta concentration, which is another metal element constituting the
crystal phase of the oxynitride, and the concentration ratio Ba/Ta
of these elements at the measurement positions 1 to 6 are shown in
Table 1 below.
[0057] As shown in Table 1, the concentration ratio between Ba and
Ta in the triple point is higher than the concentration ratio
between Ba and Ta in the crystal grain.
[0058] In addition, as shown in FIG. 4, in the sintered body
according to the present invention, a portion where the crystal
grains are in surface contact with each other preferably exists.
Here, the surface contact refers to a portion where adjacent
crystal grains are not in point contact but are in contact with
each other with a certain length in a field of view observed with
an electron micrograph. Preferably, the grain boundary portion
where the crystal grains are in surface contact with each other
does not have an amorphous phase. According to the method for
producing a sintered body according to the present invention, as
will be described later, the proportion of the sintering aid
containing cyanamide is small, and sintering is performed by
applying a mechanical pressure, so that the sintering aid
preferably does not constitute an amorphous phase. Therefore, the
density of the oxynitride in the sintered body is more effectively
enhanced, and the substantial denseness is effectively
enhanced.
[0059] In addition, the equivalent circle diameter of the voids
between the crystal grains is preferably 1.0 .mu.m or less. Here,
the equivalent circle diameter refers to the diameter of a circle
when a freely selected cross section of the sintered body is
observed with an electron microscope and a circle equal to the area
of a void between crystal grains is assumed. That is, the diameter
of a circle having the same area as the void is defined as the
equivalent circle diameter. When the equivalent circle diameter is
1.0 .mu.m or less, the size of the void is very small. Therefore,
the denseness is more effectively enhanced.
[0060] In addition, preferably, the equivalent circle diameter of a
region excluding the voids in a portion other than the crystal
grains is also 1.0 .mu.m or less. This region is considered to be
formed by once melting a sintering aid containing cyanamide and
then solidifying the sintering aid again. In this case, the
denseness of the sintered body can be further enhanced.
[0061] In the sintered body according to the present invention, the
degree of densification of the oxynitride grains in the sintered
body is preferably 55% or more, more preferably 80% or more. Here,
the degree of densification of the oxynitride grains refers to the
content of the oxynitride in the sintered body. This degree of
densification can be determined by the following method.
[0062] Method for calculating the degree of densification: The
volume and mass of the obtained sintered body are measured, and the
density is measured from the measurement results. This density is
compared with the theoretical density described, for example, in
the ICDD data. That is, the degree of densification is represented
by (determined density/theoretical density).times.100(%).
[0063] In the sintered body according to the present invention,
preferably, 90% or more of the crystal phase of the oxynitride
generally has a perovskite structure represented by AB(O,N).sub.3.
That is, the sintered body is preferably a sintered body mainly
composed of a perovskite phase. Here, A and B are metal elements
constituting the oxynitride. Here, when the composition of the
oxynitride having a perovskite structure is represented by
A.sub.aB.sub.bO.sub.oN.sub.n, a.gtoreq.b and n.ltoreq.o/2 are
preferably satisfied. The sintered body according to the present
invention may contain a composite metal oxynitride phase having a
crystal structure different from the perovskite structure,
generally represented by A.sub.m+1BO.sub.m+2N where m is an integer
of 1 or more, for example, A.sub.2BO.sub.4-xNx where x is greater
than 0 in a range of 0 wt % to 10 wt %.
[0064] Further, in the sintered body according to the present
invention, desirably, the sintered body does not contain a
crystalline metal oxide phase, a metal carbide phase, or a metal
nitride phase therein. That is, it is desirable not to have a
crystal phase that is a heterogeneous phase other than the
oxynitride phase. In this case, it is possible to obtain a sintered
body that further exhibits the characteristics of the metal
oxynitride.
[0065] The constituent element of the composite metal oxynitride is
not particularly limited, but Ta, niobium (Nb), or the like can be
used. When the sintered body is used as a dielectric, Ta is
desirably used.
[0066] In the sintered body according to the present invention,
preferably, the difference between the maximum value and the
minimum value in a wavelength range of 400 nm to 800 nm is
desirably 10% or more in the diffuse reflection spectrum in the
sintered body. In this case, the band gap is in the visible region.
The upper limit of the difference between the maximum value and the
minimum value of the reflection spectrum is not particularly
limited, but is 50% or less.
[0067] In the diffuse reflection spectrum of the powder obtained by
pulverizing the sintered body according to the present invention,
the difference between the maximum value and the minimum value in a
wavelength range of 400 nm to 800 nm is preferably 10% or more. In
this case, the powder exhibits a color tone such as red, orange, or
yellow. Therefore, it is found that the denseness of the metal
oxynitride is reliably enhanced in the sintered body. The upper
limit of the difference between the maximum value and the minimum
value of the reflection spectrum is not particularly limited, but
is 50% or less.
[0068] The direct-current volume resistance in the sintered body
according to the present invention is preferably 10.sup.6 .OMEGA.cm
or more.
[0069] The fact that the sintered body according to the present
invention is excellent in characteristics as a dielectric is
considered to be because O and N are sufficiently contained in the
sintered body, and the lattice defect of O or N is adequately
small, so that the insulating properties are improved. As explained
in relation to the Examples described later, the sintered body
obtained in the present invention exhibits orange to red almost
similarly to general oxynitride powders. This indicates that
desorption of N hardly occurs, and a sintered body having a large N
content is obtained.
[0070] In the method for producing a sintered body according to the
present invention, sintering is performed in an atmosphere
containing N.sub.2 or a rare gas or in a reduced-pressure
atmosphere of 10 Pa or less while further applying a mechanical
pressure, in a state where at least a metal oxynitride as a main
component and a sintering aid containing cyanamide are mixed.
Thereby, a sintered body with enhanced denseness can be
obtained.
[0071] As the sintering aid containing cyanamide, a sintering aid
having a melting point lower than a temperature at which a part of
N is desorbed from the metal oxynitride is preferable. Such a
sintering aid further prevents desorption of N during sintering.
The cyanamide is not particularly limited, but BaCN.sub.2,
strontium cyanamide (SrCN.sub.2), calcium cyanamide (CaCN.sub.2),
or the like can be preferably used, and BaCN.sub.2 is more
preferably used.
[0072] The BaCN.sub.2 is used as a sintering aid, and the crystal
structure of BaCN.sub.2 is not observed in the obtained sintered
body.
[0073] Preferably, the sintering aid is in the form of a powder or
particles. In that case, the metal oxynitride can be easily mixed,
and the sintering can be performed in a mixed state. Therefore, a
sintered body having a high N content can be obtained more
reliably.
[0074] In the production method of the present invention, the metal
oxynitride is preferably a material that dissolves in a liquid
phase in which cyanamide has been melted. This makes it possible to
more reliably provide a sintered body having a high nitrogen
content.
[0075] In the production method of the present invention, one
selected from the group consisting of BaTaO.sub.2N, SrTaO.sub.2N,
CaTaO.sub.2N, and LaTaON.sub.2 is preferably used as the metal
oxynitride.
[0076] An embodiment of a production method when BaCN.sub.2 is used
as a sintering aid and SrTaO.sub.2N is used as a metal oxynitride
will be described.
[0077] The melting point of BaCN.sub.2 is around 900.degree. C.
Meanwhile, the temperature at which weight change involving partial
desorption of N from SrTaO.sub.2N starts is around 1,000.degree. C.
Therefore, BaCN.sub.2 present around SrTaO.sub.2N changes to a
liquid phase at a temperature around 900.degree. C. at which
desorption of N from SrTaO.sub.2N hardly occurs. The SrTaO.sub.2N
grains are repeatedly dissolved in BaCN.sub.2 in the liquid phase
and reprecipitated. That is, SrTaO.sub.2N is sintered by liquid
phase sintering. The SrTaO.sub.2N grains are repeatedly dissolved
and reprecipitated to be bonded to each other, and grain growth
proceeds. As a result, a sintered body of
Sr.sub.1-xBa.sub.xTaO.sub.2N can be obtained.
[0078] Conventionally, sintering of SrTaO.sub.2N requires a high
temperature of 1,400.degree. C. or higher. On the other hand, in
the production method of the present invention, sintering can be
performed at a low temperature of about 880.degree. C. to
950.degree. C. Therefore, partial desorption of N hardly occurs at
the time of sintering. As a result, it is possible to obtain a
sintered body in which the N content is maintained.
[0079] As described above, the temperature at which N is desorbed
from SrTaO.sub.2N is around 1,000.degree. C. Therefore, in the
production method of the present invention, sintering is preferably
performed at a temperature lower than 1,000.degree. C. More
preferably, in the method for producing a sintered body according
to the present invention, sintering is preferably performed at a
temperature of 880.degree. C. to 950.degree. C. in a state where
the metal oxynitride and the sintering aid containing cyanamide are
in contact with each other. Within this temperature range, the
sintering aid containing cyanamide melts to proceed liquid phase
sintering, and desorption of N further hardly occurs.
[0080] In the method for producing a sintered body according to the
present invention, an aspect in which the metal oxynitride and the
sintering aid containing cyanamide are brought into contact with
each other in the sintering is not particularly limited. The metal
oxynitride and the sintering aid containing cyanamide may be mixed.
Alternatively, the sintering aid containing cyanamide may be
disposed on the metal oxynitride or under the metal oxynitride.
[0081] In the method for producing a sintered body according to the
present invention, sintering is performed in an atmosphere
containing nitrogen or a rare gas such as argon gas or in a
reduced-pressure atmosphere of 10 Pa or less while further applying
a mechanical pressure. The method for applying a mechanical
pressure is not particularly limited, and examples thereof include
an appropriate method such as pressing. For example, the metal
oxynitride and the sintering aid containing cyanamide may be
disposed in a cylindrical die, and sintering may be performed while
pressurizing from above and below using a punch or the like. In
this case, preferably, a filling powder composed of boron nitride
(BN), a metal oxynitride to be sintered, or the like may be
disposed between the punch and a raw material. The cyanamide in the
raw material melts and penetrates the raw material powder layer by
a capillary phenomenon, and further seeps out of the powder layer.
When the oozing cyanamide diffuses into the punch, there is a
possibility that the punch may be deteriorated, or elements
constituting the punch may be dissolved in the molten cyanamide and
diffuse to the raw material powder layer side. Depending on the
material constituting the punch, there is a risk of diffusion to
the raw material powder side regardless of the action of the molten
cyanamide. In order to prevent diffusion of these molten cyanamide
into the punch and diffusion of the punch material into the raw
material powder, it is desirable to use the above filling
powder.
[0082] When an atmosphere containing oxygen such as air and having
a pressure of 10 Pa or more is used as an atmosphere during
production, oxidation of the metal oxynitride or the sintering aid
containing cyanamide occurs, which is not preferable. The upper
limit of the oxygen partial pressure in the atmosphere is not
clear, but a nitrogen or rare gas atmosphere supplied from a
general gas cylinder or gas plant are suitable, and there is no
problem even if oxygen corresponding to impurities is contained in
the atmosphere.
[0083] In the method for producing a sintered body according to the
present invention, since a mechanical pressure is applied, the
amount of the sintering aid to be used may be small. That is, the
sintering aid is preferably used in a proportion of 10 wt % or less
with respect to 100 wt % of the metal oxynitride. The lower limit
thereof is preferably 3 wt % or more. Within this preferred range,
a dense sintered body can be obtained, and the amorphous phase
caused by the sintering aid can be reduced. Therefore, the
proportion of the crystal phase composed of the metal oxynitride
can be more effectively increased.
[0084] In the method for producing a sintered body according to the
present invention, the retention time at the maximum heating
temperature during sintering is preferably 1 minute to 10 minutes.
A dense sintered body can be obtained in such a relatively short
time. Heating with this preferable retention time makes it possible
to reliably provide a dense sintered body in which partial
desorption of N in the oxynitride hardly occurs.
[0085] The temperature raising rate during sintering is preferably
50.degree. C./min to 100.degree. C./min. With this preferable
temperature raising rate, temperature control is easy, and partial
desorption of N in the oxynitride hardly occurs.
[0086] The dielectric composition according to the present
invention is a dielectric composition containing the sintered body
of the present invention, and has a relative permittivity of 200 or
more when an electric field of 100 Hz to 1 MHz is applied in a
temperature range of -50.degree. C. to 150.degree. C. Therefore, a
sintered body having more excellent dielectric properties can be
provided. Accordingly, the dielectric composition according to the
present invention can be suitably used for, for example, a
capacitor. In addition, the dielectric composition containing the
sintered body according to the present invention preferably has a
rate of change in relative permittivity within .+-.10% when an
electric field of 100 kHz is applied due to a temperature change in
a temperature range of 30.degree. C. to 150.degree. C. In this
case, a capacitor or the like having a small change in relative
permittivity due to a temperature change can be provided.
[0087] The structure of the capacitor according to the present
invention is not particularly limited as long as the capacitor
includes the dielectric composition according to the present
invention and a pair of electrodes facing to each other with the
dielectric composition interposed therebetween. In this case, one
electrode of the pair of electrodes may be provided on a certain
surface of the dielectric composition, and the other electrode may
be provided on the other surface of the dielectric composition.
Alternatively, a pair of electrodes may be provided on the same
surface of the dielectric composition with a gap therebetween.
[0088] Each of the photocatalytic composition, the photoelectric
conversion element, and the gas sensor according to the present
invention includes the sintered body according to the present
invention.
[0089] In the photocatalyst, the band gap is preferably in a
visible light region, that is, in a region of 1.65 eV to 3.26 eV.
This is because the energy width of the available sunlight
increases, so that the photocatalytic properties can be enhanced.
The sintered body obtained by the present invention exhibits a
color tone of orange to red. Therefore, it is presumed that the
band gap is in the visible light region. Therefore, the sintered
body according to the present invention can be suitably used for a
photocatalytic composition that responds to visible light.
[0090] Also in a photoelectric conversion element such as a solar
cell, the photoelectric conversion material desirably has a band
gap in the visible light region, and preferably contains less
impurities. Therefore, the sintered body of the present invention
can be suitably used for the photoelectric conversion element. In
addition, the element exposed on the outermost surface of the
sintered body may contain N unlike the conventional oxide sintered
body. This provides a possibility that the reaction to the
contacted gas is different from that of the oxide. Therefore, a gas
sensor capable of measuring a gas that cannot be measured by a
conventional oxide sensor can also be provided.
[0091] Hereinafter, the present invention will be described in more
detail by giving specific examples and comparative examples.
Example 1
[0092] 1. Synthesis of BaTaO.sub.2N Powder
[0093] A barium carbonate (BaCO.sub.3) powder and a tantalum oxide
(Ta.sub.2O.sub.5) powder in an amount of 1/2 mol with respect to
BaCO.sub.3 were mixed in an acetone dispersion medium. After the
mixture was dried in air, the obtained mixed powder was placed on a
boat made of aluminum oxide (Al.sub.2O.sub.3) and the boat was
placed in a tubular furnace having a quartz glass furnace core
tube. The mixed powder was heated at 930.degree. C. for 30 hours
while ammonia (NH.sub.3) gas was allowed to flow in the furnace
core tube at a flow rate of 100 ml/min, thereby synthesizing the
BaTaO.sub.2N powder. At this time, the temperature raising rate and
the temperature lowering rate in the temperature controller of the
tubular furnace were set to 5.degree. C./min. The obtained powder
was subjected to crystal analysis using an XRD apparatus. The
results showed that the crystal phase of the obtained powder
matched the data of the inorganic crystal structure of BaTaO.sub.2N
(ICDD78-1455).
[0094] 2. Synthesis of BaCN.sub.2 Powder
[0095] A barium carbonate (BaCO.sub.3) powder was placed on an
Al.sub.2O.sub.3 boat, and the boat was placed in the same tubular
furnace used for the synthesis of the BaTaO.sub.2N powder. The
powder was heated at a temperature of 900.degree. C. for 10 hours
while NH.sub.3 gas was allowed to flow in the furnace core tube at
a flow rate of 50 ml/min. The temperature raising rate and the
temperature lowering rate were set to 5.degree. C./min.
[0096] The obtained powder was subjected to crystal analysis using
an XRD apparatus. As a result, the obtained XRD pattern was
identical to the XRD pattern obtained using the XRD apparatus in
Example 1 of Patent Document 1 (WO 2018/173491). The result
indicates that the BaCN.sub.2 powder was obtained.
[0097] 3. Preparation of BaTaO.sub.2N--BaCN.sub.2 Mixed Powder
[0098] To 100 parts by weight of the BaTaO.sub.2N powder thus
obtained, 10 parts by weight of the BaCN.sub.2 powder was added.
The mixed powder was placed in a yttria stabilized zirconia (YSZ)
container together with hexane (C.sub.6H.sub.14) as a dispersion
medium and YSZ balls, and the container was sealed. Then, the
powder was mixed using a planetary ball mill. The resulting mixed
powder was dried in a glove box in a N.sub.2 gas atmosphere. The
dried mixed powder was subjected to combustion analysis, and the O
content and the N content were determined. As a result, the O
content was 7.7 wt %, and the N content was 3.8 wt %.
[0099] The mixed powder prepared as described above was introduced
into a graphite die (hereinafter, referred to as graphite mold).
The inner diameter of the die was 10 mm. Graphite punches are
inserted into the graphite mold from above and below. Thereby, a
mechanical pressure can be applied from above and below. The powder
in the die had a three-layer structure of 150 mg of the
BaTaO.sub.2N powder, 500 mg of the mixed powder, and 150 mg of the
BaTaO.sub.2N powder. The upper and lower BaTaO.sub.2N powders are
filling powders, that is, sacrificial layers. In order to prevent
diffusion of impurities due to contact of the graphite punches with
the mixed powder, the BaTaO.sub.2N powders were disposed.
[0100] The graphite mold was set in a pressure sintering apparatus,
the pressure was reduced to about 6 Pa, and a pressure of 70 MPa
was applied to the mixed powder. That is, the above described
pressure was applied to the mixed powder by the graphite punches.
At this time, in order to apply a surface pressure of 70 MPa to a
circular portion having a diameter of 10 mm, the output of the
pressure apparatus was set to about 5.5 kN. The mixed powder was
heated while this pressure was applied. The temperature raising
rate was set to 50.degree. C./min, and the temperature was raised
to a temperature of 900.degree. C. After retaining the temperature
at 900.degree. C. for 3 minutes, the heating was stopped and the
mixed powder was naturally cooled in the pressure sintering
apparatus. The pressure was reduced to 0.6 kN (surface pressure:
about 10 MPa) about 3 minutes after the start of the natural
cooling.
[0101] After confirming that the temperature of the graphite mold
decreased to around room temperature, the graphite mold was taken
out from the pressure sintering apparatus, and then the sintered
body was released from the graphite mold. Sacrificial layers
derived from the BaTaO.sub.2N powder adhered above and below the
sintered body. The upper and lower sacrificial layers were removed
by grinding using abrasive paper. The obtained sintered body was a
hard sintered body having a reddish brown color tone and a disk
shape. The diameter, thickness, and mass of the obtained
disk-shaped sintered body were measured. The density was measured
from the measurement results. This density was compared with the
theoretical density of BaTaO.sub.2N described in the ICDD data. As
a result, the obtained density was 83.0% of the theoretical
density, that is, the degree of densification, which is the
relative density, was 83.0 wt %.
[0102] The sintered body was subjected to combustion analysis, and
the result showed that the O content was 7.9 wt %, and the N
content was 3.7 wt %.
[0103] The surface of the sintered body was ground with abrasive
paper having silicon carbide (SiC) abrasive grains, and XRD
measurement and X-ray fluorescence (XRF) measurement were
performed. XRD measurement was performed at each depth direction
position from the surface to the bottom surface of the disk-shaped
sintered body. FIG. 1 is a graph showing an XRD pattern at each
depth position from a surface to a bottom surface of the sintered
body obtained in Example 1.
[0104] As is clear from FIG. 1, only the BaTaO.sub.2N phase and the
Ba.sub.2TaO.sub.3N phase were detected from the surface of the
sintered body to the central position in the depth direction. In
addition, diffraction peaks of oxides, carbides, and carbonates
were not observed. The diffraction derived from SiC of abrasive
grains of the abrasive paper and silicon clay for sample fixation
in the XRD analysis was observed.
[0105] The abundance of each phase was estimated from each peak
intensity ratio between the BaTaO.sub.2N phase and the
Ba.sub.2TaO.sub.3N phase appearing in the XRD pattern. As a result,
the content of BaTaO.sub.2N was about 94.16 wt %, and the content
of Ba.sub.2TaO.sub.3N was 5.84 wt %.
[0106] In addition, the molar ratio of Ba to Ta was calculated by
XRF measurement and found to be about 53:47. That is, the result
confirmed that the sintered body was basically a Ba-rich sintered
body.
[0107] The outer peripheral side portion of the sintered body and
the inside thereof, exposed due to breaking were observed by the
SEM. FIG. 2 is an SEM photograph of the outer peripheral side
portion of the sintered body, and FIG. 3 is an SEM photograph of
the inside of the sintered body, both photographs being captured at
a magnification of 5,000 times. As is clear from FIG. 2 and FIG. 3,
no structural difference was observed between the outer peripheral
side portion and the inside. In addition, there was no void to the
extent that the size, that is, the diameter can be measured.
[0108] The inside of the obtained sintered body was observed with a
TEM. FIG. 4 is a HAADF-STEM photograph at a magnification of
160,000 times, and FIG. 5 is a photograph showing a bright field
image of a TEM photograph.
[0109] As is apparent from FIG. 4 and FIG. 5, it is found that
crystal grains of the oxynitride had a grain size of about 100 nm
to 200 nm. In addition, the crystal grains are deformed, brought
into close contact with each other, and bonded in a surface contact
state. Here, the surface contact state is not a point contact but a
linear portion where adjacent crystal grains are in contact with
each other in the cross section of the sintered body. That is, the
surface contact state means that contact is made on a surface
having a certain area.
[0110] The length of the linear portion is not particularly
limited, but as shown in FIG. 4 and FIG. 5, the surface contact
means that when the cross section of the sintered body is observed
at a magnification at which 10 to 100 grains can be distinguished,
a portion where the contours of the grains in contact with each
other overlap is observed as a line rather than a point. As shown
in FIG. 2 and FIG. 3, when a large number of more than 100 grains
are included in the field of view and the state of bonding between
individual grains cannot be distinguished, the presence of surface
contact should not be verified. Conversely, in an observation image
at a significantly high magnification in which only a contact
portion between two grains is enlarged, even if the contact portion
is observed as a line, it cannot be said that the grains are in
surface contact unless the grains are in contact with each other at
a position not included in the field of view.
[0111] In addition, FIG. 4 shows that the size of the voids between
the crystal grains is also about 100 nm to 200 nm. Further, a thin
skin-like pattern having a thickness of about 10 nm is observed on
the surfaces of the crystal grains in contact with the voids. As
shown in FIG. 5, bright color tone portions were present, and a
void having a shape close to a circular shape was observed at each
portion.
[0112] FIG. 6 is a HAADF-STEM photograph of the sintered body
obtained in Example 1, and is a photograph showing positions where
the concentration ratio between Ba and Ta was measured.
[0113] Reference numerals 1 to 6 in FIG. 6 indicate a measurement
position. The measurement position 4 is a void portion where three
crystal grains are adjacent, that is, the triple point. Energy
dispersive X-ray (EDX) analysis was performed at a portion forming
the triple point, that is, the measurement position 4 and at the
measurement positions 1 to 3, 5, and 6 of the peripheral edge
thereof. The Ba concentration, the Ta concentration, and the Ba/Ta
ratio at each measurement position are shown in Table 1 below.
TABLE-US-00001 TABLE 1 Measurement Ba Ta position at % at % Ba/Ta 1
1.1 1.3 0.87 2 1.0 0.9 1.03 3 1.0 0.9 1.16 4 0.9 0.5 1.90 5 0.6 0.6
1.07 6 0.9 1.1 0.78
[0114] As is apparent from Table 1, the Ba/Ta ratio was the highest
in the grain boundary portion, that is, at the triple point. In
addition, with respect to the Ba content at the measurement
position 4, there were a case where the Ba content at the
measurement positions 1, 2, 3, 5, and 6 inside the crystal grains
was larger than the Ba content at the measurement position 4 and a
case where the Ba content at the above measurement positions was
smaller than the Ba content at the measurement position 4. On the
other hand, with respect to the Ta content, the Ta content at the
measurement position 4 was the smallest.
[0115] TEM bright field images were taken at a plurality of
portions in the sintered body at a magnification of 150,000 times.
FIG. 7 to FIG. 11 are photographs showing TEM bright field images
of the sintered body.
[0116] The grain size distribution, the void distribution, and the
distribution of the region excluding voids in the portion other
than crystal grains were measured using image analysis software
"A-Zou Kun" (manufactured by Asahi Kasei Engineering Corporation)
in the same manner as the analysis method described in Patent
Document 1. Here, the region excluding voids in the portion other
than crystal grains is a portion considered to be an amorphous
phase generated when BaCN.sub.2 melted once during sintering and
then solidified again. The measurement method is the same as the
method described in Patent Document 1, and the analysis method and
the measurement method are incorporated herein.
[0117] As a result, 292 grains were measured in the grain size
distribution. The equivalent circle diameter of the crystal grain
was 120.2 nm. Further, in the measurement of the void distribution,
83 voids were measured, and the circle equivalent average diameter
of the voids was 81.0 nm. The number of portions considered to be
the amorphous phase was 99, and the circle equivalent average
diameter of the portions was 69.5 nm.
[0118] FIG. 12 is a graph showing the distribution of equivalent
circle diameter of crystal grains. FIG. 13 is a graph showing the
distribution of equivalent circle diameter of voids. FIG. 14 is a
graph showing the distribution of the equivalent circle diameter of
portions considered to be an amorphous phase.
[0119] On the other hand, the sintered body was subjected to SEM
observation at magnifications of 300 times and 5,000 times. FIG. 15
is an SEM photograph of the sintered body obtained in Example 1
observed at a magnification of 300 times. FIG. 16 is an SEM
photograph of the sintered body obtained in Example 1 observed at a
magnification of 5,000 times.
[0120] In both FIG. 15 and FIG. 16, no void exceeding a diameter of
10 .mu.m was observed. In the SEM photograph of Patent Document 1
observed at a magnification of 300 times shown in FIG. 3, there
were a large number of voids having a diameter of several tens of
.mu.m, whereas in the sintered body obtained in Example 1, the size
of the voids was very small and about the same as the size of the
oxynitride grains.
[0121] In FIG. 5 and FIG. 6 of Patent Document 1, there were
regions of an amorphous phase having a diameter of several hundred
nm. On the other hand, in Example 1, the size of the region
considered to be an amorphous phase was also about the same as the
size of the oxynitride grains.
[0122] As shown in FIG. 5 and FIG. 6 of Patent Document 1, the
sintered body described in Patent Document 1 had a structure
similar to a complex in which oxynitride grains are confined in a
large amount of the amorphous phase. On the other hand, in the
sintered body obtained in Example 1, portions where oxynitride
grains are in close contact with each other without interposing the
amorphous region occupy almost all of the sintered body. That is,
it is found that there are many regions where the amorphous phase
does not exist between the crystal grains, and as a result, the
substantial degree of densification of the sintered body is
effectively enhanced. In addition, since the sizes of the voids and
the amorphous phase are equal to the size of the oxynitride grains,
it is considered that the sizes of the voids and the amorphous
phase also change depending on the size of the oxynitride raw
material main body to be used. This phenomenon is considered to be
caused by the fact that when sintering is performed by applying a
mechanical pressure, sintering proceeds while the oxynitride grains
are repeatedly rearranged due to the liquid phase of molten
BaCN.sub.2 and the pressure, leading to densification. It is
considered that the oxynitride grains enter the void portion which
is sufficiently larger than the oxynitride grains, and thus the
size of the voids is reduced to the same extent as that of the
oxynitride grains. In addition, BaCN.sub.2 which has melted into
the liquid phase remains in the void portion and then solidifies
due to a temperature drop after completion of the sintering.
Therefore, the size of the amorphous solidified product derived
from BaCN.sub.2 is considered to be about the same as that of the
oxynitride grains. In addition, it is considered that, among
BaCN.sub.2 which has melted into the liquid phase, BaCN.sub.2
distributed in the vicinity of the surface of the BaTaO.sub.2N
grains dissolves BaTaO.sub.2N to form a crystalline layer of
Ba.sub.2TaO.sub.3N.
[0123] The size of the oxynitride grains was about 100 nm in
Example 1, but can be increased to about 1.0 .mu.m by adjusting the
composition and the production method. However, when a sintered
body is produced using oxynitride grains having a size as large as
1.0 .mu.m or more as a raw material, the void portion similarly
increases to 1.0 .mu.m or more, and it may be difficult to utilize
the electrical characteristics of the oxynitride, or the mechanical
strength may decrease.
[0124] (Diffuse Reflectance)
[0125] The sintered body obtained in Example 1 was pulverized using
an agate mortar to obtain a powder. The obtained powder was
measured for diffuse reflectance in the visible light region (400
nm to 800 nm).
[0126] FIG. 17 is a graph showing the diffuse reflection spectrum
of a powder obtained by pulverizing the sintered body obtained in
Example 1.
[0127] As is clear from FIG. 17, the minimum value of the
reflectance in the visible light region is about 13%, and the
maximum value thereof is about 26%.
[0128] In the black sintered body of the reference example,
sintered at a high temperature of 1,400.degree. C., the diffuse
reflectance in the visible light region was almost constant at 10%
to 13%. That is, the light absorption edge could not be
confirmed.
[0129] (Volume Resistance)
[0130] Platinum (Pt) films were deposited on the upper surface and
the lower surface of the sintered body obtained in Example 1. Then,
the volume resistance at a temperature of -50.degree. C. to
150.degree. C. was measured at each frequency in a range of 100 Hz
to 1 MHz. The results are shown in FIG. 18.
[0131] FIG. 18 is a graph showing the relationship between the
temperature and the volume resistance of the sintered body of
Example 1 at a plurality of frequencies in a range of 100 Hz to 1
MHz.
[0132] As is clear from FIG. 18, the volume resistance was on the
order of 10.sup.7 .OMEGA.cm at 100 Hz, but the volume resistance
decreased as the frequency increased. The volume resistance was on
the order of 10.sup.3 .OMEGA.cm at 1 MHz. With respect to the
temperature, inflection points were slightly observed in the
vicinity of 60.degree. C. and the vicinity of 100.degree. C. only
at 10 kHz.
[0133] (Dielectric Properties)
[0134] The dielectric properties of the sintered body obtained in
Example 1 were measured at a frequency of 100 kHz in a temperature
range of 25.degree. C. to 160.degree. C. The results are shown in
FIG. 19 and FIG. 20. FIG. 19 is a graph showing the relationship
between the temperature and the relative permittivity of the
sintered body obtained in Example 1. FIG. 20 is a graph showing the
relationship between the temperature and the dielectric loss tan
.delta. of the sintered body obtained in Example 1.
[0135] As can be seen from FIG. 19, the value of the relative
permittivity was between 300 and 380 in the above temperature
range. The value of tan .delta. was between 0.08 (8%) and 0.17
(17%) in this temperature range.
[0136] FIG. 21 is a graph showing the relationship between the
temperature and the relative permittivity measured in a frequency
of 100 Hz to 1 MHz and at temperatures of -50.degree. C.,
50.degree. C., and 150.degree. C., and FIG. 22 is a graph showing
the relationship between the temperature and the dielectric loss
tan .delta..
[0137] As is apparent from FIG. 21, the relative permittivity is
200 to 850 in this frequency range. In the sintered body of Example
1 described in Patent Document 1, the relative permittivity was
only about several tens to 200. Therefore, it is found that the
relative permittivity of the sintered body obtained in Example 1 is
remarkably high. This is considered to be because the proportion of
the oxynitride in the sintered body is increased.
[0138] On the other hand, as shown in FIG. 22, when measurement was
performed in this frequency range, tan .delta. fell within a range
of 0.05 (5%) to 0.35 (35%). The tan .delta. was equivalent to that
of the sintered body of Example described in Patent Document 1.
Consequently, in the sintered body of Example 1, the relative
permittivity is increased, and on the other hand, the tan .delta.
is suppressed to be low.
[0139] (Characteristics of Sintered Body Obtained in Example 1)
[0140] As described above, it is considered that the oxynitride
grains were partially dissolved in the molten BaCN.sub.2 by
performing sintering while applying a mechanical pressure, and as a
result, the volume of the crystal phase was reduced. Therefore, it
is considered that rearrangement of crystal grains and
densification of the entire powder phase were promoted.
[0141] In addition, the O content and the N content in the mixed
powder before sintering were substantially equal to the O content
and the N content in the sintered body. In the conventional method
for sintering an oxynitride, a large amount of N is desorbed along
with thermal decomposition, and anion deficiency may occur, or
different phases such as an oxide phase and a carbide phase may be
generated. Therefore, the absorption rate in the visible light
region was increased, and the sintered body was changed to
black.
[0142] On the other hand, the sintered body obtained in Example 1
exhibited a reddish brown color. In addition, a variation in
reflectance in the visible light region was observed in the diffuse
reflection spectrum. That is, the result indicates that a band gap
exists in the visible light region. The presence of the band gap in
the visible light region is one of characteristics of the
perovskite-type oxynitride. Therefore, it is possible to provide a
sintered body suitable for applications such as the above-described
photocatalyst and solar cell.
[0143] Furthermore, in the sintered body obtained in Example 1, the
amount of BaCN.sub.2 added can be reduced by performing sintering
while applying a mechanical pressure. Therefore, remaining of large
voids was suppressed. In addition, after BaCN.sub.2 was dissolved,
the amount of BaCN.sub.2 gradually volatilized is also decreased.
Therefore, in Patent Document 1, as described above, a large number
of large voids remained, whereas in Example 1, the size of voids
was equivalent to that of the oxynitride grains. This is considered
to be because rearrangement of crystal grains proceeded and
volatilization of BaCN.sub.2 was suppressed.
[0144] In the sintered body obtained in Example 1, the proportion
of the oxynitride phase in the sintered body is high. This is
considered to be because BaCN.sub.2 gradually oozes out of the
green compact layer and is discharged by performing sintering while
being pressurized. Also due to this effect, it is considered that
not only the proportion of the oxynitride grains in the sintered
body is increased, but also the volume of the amorphous phase
derived from BaCN.sub.2 is significantly decreased.
[0145] As described above, the fact that the Ba content in the
triple point is high and that a small amount of Ta is contained in
the triple point as compared with the inside of the crystal grain
means that the liquid phase component of molten BaCN.sub.2 remains
in the triple point and solidifies, and that BaTaO.sub.2N is
dissolved in the liquid phase of BaCN.sub.2. Such a region having a
high Ba concentration and a low Ta concentration was not observed
on the contact surface between the grains, and was observed only at
the triple point, that is, the minimum void portion. Therefore,
BaCN.sub.2 can be added only in an amount necessary for sintering
the liquid phase of the oxynitride phase. In addition, it is
considered that excessive BaCN.sub.2 is discharged to the outside
of the green compact layer with pressurization. It is also
considered that the oxynitride grains are brought into contact with
each other by sufficient pressurization, and the oxynitride grains
are further deformed accompanying dissolution and reprecipitation.
As a result, it is considered that the oxynitride grains are in
surface contact with each other as described above.
[0146] In Example 1 described above, no peaks of oxides, carbides,
nitrides, and carbonates were observed in the XRD pattern of the
sintered body. The result indicates that the purity of the
oxynitride in the sintered body obtained in Example 1 is high.
[0147] In the sintered body obtained in Example 1, in which the
number of voids is small, the proportion of the oxynitride phase is
high, and no anion deficiency occurs, the characteristics that the
oxynitride phase essentially has can be effectively utilized. For
example, the volume resistance at room temperature and 100 kHz is
as high as about 30 Mom. In addition, as described above, the
relative permittivity is remarkably increased as compared with the
sintered body obtained in Examples described in Patent Document 1,
and the dielectric loss is also reduced to about 10 to 20%.
Therefore, the sintered body obtained in Example 1 is more
effective as a dielectric ceramic material, that is, a capacitor
application than the sintered body of Examples described in Patent
Document 1. In addition, since the proportion of the oxynitride
phase is high, it can be suitably used for a photocatalyst, a solar
cell, a gas sensor, or the like.
Examples 2 to 12, Comparative Examples 1 to 4
[0148] Production of the sintered bodies of Examples 2 to 12 and
Comparative Examples 1 to 4 was attempted by changing the
composition of the used oxynitride powder, the amount of BaCN.sub.2
added, the pressure during sintering, the atmosphere, the sintering
temperature, the temperature raising rate, and the retention time
as shown in Table 2 below.
TABLE-US-00002 TABLE 2 Degree of BaCN.sub.2 Sintering Temperature
Retention densification of Composition content Pressure temperature
raising rate time Solidified oxynitride phase Anion of oxynitride
wt % MPa .degree. C. .degree. C./min min or not % deficiency
Example 2 BaTaO.sub.2N 5 30 900 50 1 Solidified 68.7 No Example 3
BaTaO.sub.2N 5 70 900 50 1 Solidified 80.7 No Example 4
BaTaO.sub.2N 5 70 900 50 3 Solidified 79.8 No Example 5
BaTaO.sub.2N 5 70 900 50 5 Solidified 84.1 No Example 6
BaTaO.sub.2N 5 100 900 50 3 Solidified 80.1 No Example 7
BaTaO.sub.2N 7 30 900 50 3 Solidified 66.5 No Example 8
BaTaO.sub.2N 7 70 900 50 3 Solidified 72.1 No Example 9
BaTaO.sub.2N 7 100 900 50 3 Solidified 76.9 No Example 10
BaTaO.sub.2N 10 30 900 50 3 Solidified 70.5 No Example 11
BaTaO.sub.2N 10 100 900 50 3 Solidified 82.4 No Example 12
SrTaO.sub.2N 10 100 900 50 10 Solidified 97.0 No Comparative
BaTaO.sub.2N 0 30 900 50 10 Green Not measurable Yes Example 1
compact-like (easily broken) Comparative BaTaO.sub.2N 0 100 900 50
10 Green Not measurable Yes Example 2 compact-like (easily broken)
Comparative BaTaO.sub.2N 10 0 900 50 1800 Solidified 54.0 Yes
Example 3 Comparative BaTaO.sub.2N 10 35 930 10 60 Solidified 77.1
Yes Example 4
[0149] The SrTaO.sub.2N powder was synthesized in the same manner
as in Example 1 described in Patent Document 1.
[0150] Results of Examples 2 to 12
[0151] In Examples 2 to 12, sintered bodies in which no anion
deficiency occurred could be obtained as in Example 1. That is,
blackening or resistance reduction of the sintered body did not
occur.
[0152] FIG. 23 is a graph showing the diffuse reflection spectra of
the sintered body obtained in Example 2 and a powder obtained by
pulverizing the sintered body. The solid line indicates the
measurement result of the powder prepared by pulverizing the
sintered body, and the broken line indicates the measurement result
of the sintered body as it is. From the spectrum of the powder, the
diffuse reflectance in the visible light region changed around
32.5% to 43.5%, and the difference between the maximum value and
the minimum value was about 11.0%.
[0153] When the composition of the oxynitride was SrTaO.sub.2N, a
good sintered body was obtained as in Example 1 as long as the
amount of BaCN.sub.2 added was 5 wt % to 10 wt %, and the retention
time was 10 minutes or less.
[0154] In Examples 2 to 12, pressure sintering was performed in the
same manner as in Example 1. Therefore, there was no large void in
the obtained sintered body, and good electrical properties were
obtained. However, the relative permittivity depends on the degree
of densification. Accordingly, when the degree of densification of
the obtained sintered body was low as in Example 7, the
permittivity was lower than that in Example 1. In addition, in the
sintered body of Examples using SrTaO.sub.2N, similar sintered
bodies were obtained even when the retention time was extended to
10 minutes. This is considered to be because SrTaO.sub.2N has a
higher temperature at which anion deficiency occurs due to partial
desorption of N than BaTaO.sub.2N. In particular, in Example 12, a
hard sintered body having an extremely high density of 97% in the
degree of densification and no anion deficiency was obtained. When
there is no anion deficiency in the sintered body made of
SrTaO.sub.2N, the color tone is orange.
Comparative Examples 1 to 4
[0155] In Comparative Examples 1 and 2 in which no BaCN.sub.2 was
added, solidification did not occur.
[0156] In Comparative Examples 3 and 4, in which the retention time
was as long as 10 minutes or more, solidification occurred, but
anion deficiency occurred.
[0157] In Comparative Example 3, in which densification was
attempted by performing sintering for a long time without
pressurization, the degree of densification of the obtained
sintered body was only 54%, and was hardly densified. Anion
deficiency also occurred.
[0158] In Comparative Example 4, the retention time was shortened
to 60 minutes instead of applying a mechanical pressure as compared
with Comparative Example 3. Although the degree of densification
was improved to 77%, anion deficiency occurred.
[0159] Comparison between Examples and Comparative Examples shows
that in order to produce a dense sintered body while avoiding the
formation of anion deficiency due to partial desorption of N of the
metal oxynitride, it is necessary to add cyanamide, heat the
sintered body while applying a mechanical pressure, and set the
retention time at the maximum heating temperature to a short time
(1 minute to 10 minutes).
* * * * *